[Mac] Fixes crash where killing an extension via the task manager while a popup is shown would hose the browser.
[Mac] Fixes crash where killing an extension via the task manager while a popup is shown would hose the browser.

BUG=32653
TEST=none
Review URL: http://codereview.chromium.org/554044

git-svn-id: http://src.chromium.org/svn/trunk/src@36878 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: fc1326d4b3e4ee7b7731a1b6f0e565893edc9a33
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser